Internal combustion engines may use variable cam timing (VCT) to increase vehicle fuel economy and emissions performance. Many variable cam timing systems use a vane type cam phaser that may be controlled by an electromechanically actuated spool valve that directs oil flow to one side or the other of the vane in order to actuate the phaser, thereby adjusting camshaft timing with respect to the engine crankshaft timing. Actuation of these variable timing systems, and thus the timing of the engine cylinder valves, is highly dependent on the oil flow and oil pressure in the VCT system. Modern oil control systems may include an oil pump, known as a variable displacement oil pump (VDOP), which may be calibrated to adjust the pump operation responsive to parameters such as engine temperature, engine load, and engine speed. These oil control systems often supply multiple engine subsystems with competing and overlapping demands, including bearings, hydraulic valve mechanisms, VCT systems, and piston cooling jets. Further, in order to minimize parasitic losses and increase fuel economy, variable displacement oil pumps are often calibrated to supply the minimum oil pressure to maintain lubrication engine components, provide sufficient cooling, and actuate engine components.
The inventors herein have recognized that by controlling the oil pressure to the VCT system based on conventional engine operating parameters, VCT phasers may not be supplied with sufficient oil pressure under certain operating conditions, namely operating conditions that necessitate an abrupt or substantial shift of the VCT phaser, to facilitate the desired phaser response. Operating the VCT system with reduced oil pressure may result in reduced phasing velocity of the VCT phasers, resulting in degraded performance of the VCT system. Degraded performance of the VCT system may include delayed engine response, a lag in turbo spool up time, as well as noise, harshness, and vibration (NVH) issues.
Other attempts to address providing the VCT system with sufficient oil pressure include incorporating an auxiliary system to allow the short duration, high flow rate pulses needed to shift the VCT phaser. One example approach is shown by Aimone in U.S. Pat. No. 6,871,620. Therein, Aimone discloses an engine system with an on-demand auxiliary oil pump used in conjunction with an accumulator, in addition to a conventional oil pump, to supply adequate oil pressure to the VCT system at very low engine speeds or when the VCT unit returns to a locked start-up position. Still further attempts to address providing the VCT system with sufficient oil pressure include upsizing a fixed rate oil pump to meet the oil pressure demands of the VCT system. Yet another attempt to provide sufficient oil pressure to the VCT system includes adjusting the oil pressure delivered to the VCT system based on a comparison of a modeled phasing rate and an actual phasing rate of the VCT, and correlating the data to the pressure in the engine oil system.
However, the inventors herein have recognized potential issues with such systems. As one example, adding an auxiliary oil system to supply the VCT system adds cost and complexity to the vehicle. Operating a supplemental pump may put additional load on the engine, thereby decreasing performance, and adding a pump and an accumulator to an already tight engine compartment may not be feasible. Upsizing a fixed rate engine oil pump to meet the periodic high demands of the VCT system is known to increase parasitic losses and degrade fuel economy. As another example, adjusting oil pressure to the VCT system based on computationally intensive models may not accurately represent actual, transient engine operating conditions. Additionally, such a system may not be capable of completing the computation rapidly enough to adjust the VCT as demanded.
In one example, the issues described above may be addressed by a method for adjusting an oil pressure supplied to an engine and a variable cam timing (VCT) system to a first level based on engine speed, engine load, and engine oil temperature; and in response to a condition of the VCT system, increasing the oil pressure to a second level that is higher than the first level for a duration. In this way, a VCT system may be supplied with sufficient oil pressure during operating conditions that include an abrupt or substantial shift of the VCT system.
As one example, the method might include, during a first condition, adjusting the output of an oil pump that supplies oil to an engine and a variable cam timing (VCT) system to a first level that is based on engine speed, engine load, and engine oil temperature. During a second condition, the method may include adjusting the output of the oil pump to a second level that is based on a commanded position of the VCT system and not engine speed, engine load, and engine oil temperature, where the second level is greater than the first level.
In this way, a variable displacement oil pump may be selectively controlled to provide an increased level of oil pressure to the VCT system during certain operating conditions of the VCT system, leading to an efficient oil delivery system that may minimize parasitic losses and increase fuel economy while still meeting the intermittent high demands of the VCT system. As a result of using the existing engine oil pump to supply the VCT system rather than adding a supplementary oil system, system costs may be reduced. By expeditiously supplying the VCT system with the requisite oil pressure for rapid shifting, issues with turbo lag and NVH may decrease, while engine responsiveness and performance may increase.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.